EP0720262B1 - Laser device with organic Bragg reflector and fabrication method - Google Patents

Description

The present invention relates to a component Bragg reflector laser, as well as a component monolithic incorporating such a component. It relates to also a method for making such a component.

The invention advantageously finds application in the field of fiber optic telecommunications and more particularly for the multiplexed links in wavelength or coherent systems.

For a few years, an unceasing interest was granted to the semiconductor laser to Bragg reflector (DBR), especially for applications in multiplexed links (WDM) and for future coherent systems. DBR lasers are indeed sources with high temporal coherence (low line width), which have a tunability of emission wavelength, which can be significant.

A Bragg reflector laser is a structure monolithic laser integrating, at the end of a medium semiconductor amplifier (active section), a guide passive optics, in which a network is produced constituting a Bragg reflector (Bragg section). The passive optical guide may have a portion devoid of network, interposed between the amplifying medium and the reflecting network, which defines on the DBR structure a phase control section. This section allows avoid skipping modes during the tunability of the DBR section.

Generally, the wavelength tunability of such a DBR laser is achieved by injecting current into the Bragg or phase adaptation sections. The injection of minority carriers makes it possible to vary the refractive index of the medium and therefore to control the wavelength λ _B of Bragg given by the formula λ _B = Λ / 2n _eff, where Λ is the pitch of the network and n is the effective index of the environment.

• "A theorical model of multielectrode DBR lasers" - X. Pan, H. Olesen, B. Tromborg - IEE - Journal of quantum electronics - Vol. 24, N° 12, december 1988.

For a more detailed presentation of DBR lasers, one can advantageously refer to the publication:

• "A theorical model of multielectrode DBR lasers" - X. Pan, H. Olesen, B. Tromborg - IEE - Journal of quantum electronics - Vol. 24, No. 12, December 1988.

Despite significant advances in techniques semiconductor components, the manufacturing a Bragg reflector laser remains complex and requires many stages of growth and engraving. The control of the coupling between the section amplifier and guide section, as well as the choice of the lateral structure of the guide and its confinement electrical and optical are of great importance for the performance of the component produced.

In addition, if the techniques of tunability by current injection have the advantage of allowing access large ranges of tunability (10 to 12 nm), they are nevertheless at the origin of an enlargement significant spectral, as well as a decrease in the emitted power. These phenomena are linked to fluctuations carrier concentration due to the emission spontaneous in the passive section.

Recently it has been proposed to tune in length wave of DBR lasers by localized heating of their passive section, this heating being obtained, either by injecting a reverse current into the guiding section, either using layer heating resistors thin.

• "A Three-Electrode Distributed Bragg Reflector Laser with 22 nm Wavelength Tuning Range" - M. Oberg et al, IEEE Photonics Technology Letters, Vol. 3, NO, 4, Avril 1991,
• "A DBR laser Tunable by Resistive Heating" - S.L. Woodward et al, IEEE Photonics Technolog Letters, Vol. 4, NO 12, Decembre 1992.

In this regard, it is advantageous to refer to the following two publications:

• "A Three-Electrode Distributed Bragg Reflector Laser with 22 nm Wavelength Tuning Range" - M. Oberg et al, IEEE Photonics Technology Letters, Vol. 3, NO, 4, April 1991,
• "A DBR laser Tunable by Resistive Heating" - SL Woodward et al, IEEE Photonics Technolog Letters, Vol. 4, NO 12, December 1992.

However, neither of these two techniques tunability by heating is not satisfactory.

In both cases, the section heating guide is not perfectly located and induces a temperature rise of the active section enhancer which results in a degradation of characteristics of the laser (higher threshold currents, lower quantum yields).

In addition, the solution implementing reverse currents generate, at the junctions damage to the guide section decrease the performance of the component.

An object of the invention is to propose a component which overcomes these various drawbacks.

To this end, the invention provides a component Bragg reflector laser comprising a substrate, which carries, on the one hand, an active transmission section in semiconductor materials and, on the other hand, a guide wave reflector extending this section active, means for varying the index of waveguide refraction, to control tunability in wavelength of said laser component, characterized in that said waveguide is made of organic material (s).

Such a component is simple to manufacture, the organic materials being easy to work: deposit and easy engraving; Bragg gratings by direct engraving or photo-inscription on the guide.

It will also be noted that the losses in the part waveguide of such a component are weak (lower at 1 dB / cm for the emission wavelengths of laser transmitters for optical telecommunications (1.3 and 1.55 µm)).

Component tunability can be achieved by thermooptical effect, which reduces the skate widening and power loss effects issued observed in the case of tunability by injection.

In this case, the Bragg grating can be him also obtained by thermooptical effect. The component then includes a network of heating resistors which induce on the waveguide an index network of refraction.

Organic materials present, in relation semiconductor materials, the advantage of being of a lower thermal conductivity, so that thermal disturbances induced by the heating of the Bragg section (and where appropriate of the section of phase control) on the active section are reduced.

The organic materials of the waveguide are especially preferably chosen so as to have a low thermal conductivity, preferably of the order of or less than 1 W / (m. ° K) (against about 70 W / (m. ° K) for semiconductors) and a coefficient (dn / dT) of variation in refractive index as a function of the high temperature, preferably greater than 10 ^-4 ° C ^-1 .

Alternatively, the component's tunability can be obtained using the electro-optical effect.

Another object of the invention is a component monolithic incorporating at least one laser component with Bragg reflector of the aforementioned type. Advantageously then at least one integrated organic optical guide extends the Bragg section of the laser component.

• on réalise, par croissance épitaxiale sur le substrat, un empilement de couches semi-conductrices correspondant à la structure de la section active ;
• on réalise, par gravure et reprise d'épitaxie, la structure latérale de cette section active ;
• on réalise, par masquage et gravure sèche jusqu'au substrat, un sillon dans la structure ainsi obtenue ;
• on dépose une couche antireflet en un matériau diélectrique dans le fond et les bords du sillon, ainsi que sur la face de sortie de la section active ;
• on dépose sur cette couche antireflet une couche de confinement en un matériau organique présentant un indice de réfraction plus faible que celui du coeur du guide ;
• on dépose sur cette couche de confinement une couche d'un matériau organique correspondant au coeur du guide;
• on réalise une gravure en ruban sur les couches organiques ainsi déposées ;
• on réalise le cas échéant le réseau de Bragg sur le ruban correspondant au coeur du guide ;
• on dépose sur le ruban et sur la couche antireflet une couche de confinement supérieure en un matériau organique présentant un indice de réfraction plus faible que celui du coeur du guide,
• on dépose sur cette couche de confinement une couche diélectrique de protection,
• on dépose sur cette couche diélectrique des couches formant résistances de chauffages.

Yet another object of the invention is a method for producing such a laser component. This process is characterized by the following stages:

• is carried out, by epitaxial growth on the substrate, a stack of semiconductor layers corresponding to the structure of the active section;
• the lateral structure of this active section is produced by etching and resumption of epitaxy;
• is carried out, by masking and dry etching to the substrate, a groove in the structure thus obtained;
• an anti-reflective layer of dielectric material is deposited in the bottom and the edges of the groove, as well as on the exit face of the active section;
• a confinement layer made of an organic material having a lower refractive index than that of the guide core is deposited on this anti-reflective layer;
• a layer of an organic material corresponding to the core of the guide is deposited on this confinement layer;
• ribbon etching is carried out on the organic layers thus deposited;
• if necessary, the Bragg grating is produced on the ribbon corresponding to the core of the guide;
• an upper confinement layer made of an organic material having a lower refractive index than that of the guide core is deposited on the tape and on the anti-reflective layer,
• a protective dielectric layer is deposited on this confinement layer,
• layers forming heating resistors are deposited on this dielectric layer.

According to an advantageous embodiment, we after the deposition of the first organic layer of confinement, an etching of said layer to release the exit face of the active section.

The different stages of such a process are of simple implementation.

It should also be noted that it is possible with such a process of characterizing and qualifying the active section in semiconductor materials before the waveguide organic is not realized. This saves time and cost compared to DBR "all semiconductor" lasers, for which the active section cannot be characterized and qualified only at the end of the manufacturing of the component.

• la figure 1 est une demi-vue en perspective d'un composant conforme à l'invention ;
• la figure 2 est une représentation schématique en coupe longitudinale du composant de la figure 1 ;
• la figure 3 est une représentation semblable à celle de la figure 2 d'une autre variante possible pour l 'invention ;
• les figures 4 et 5 représentent schématiquement deux composants monolithiques intégrant un composant selon l'invention.

Other characteristics and advantages of the invention will emerge from the description which follows. This description is purely illustrative and not limiting. It must be read in conjunction with the appended drawings in which:

• Figure 1 is a half perspective view of a component according to the invention;
• Figure 2 is a schematic representation in longitudinal section of the component of Figure 1;
• Figure 3 is a representation similar to that of Figure 2 of another possible variant for the invention;
• Figures 4 and 5 schematically represent two monolithic components incorporating a component according to the invention.

The laser component shown in Figures 1 and 2 has been generally referenced by 1 and includes a laser amplifier section 2 extended by a guide waveguide 3. This waveguide 3 has a Bragg section 4 and a phase 5 control section interposed between the amplifier section 2 and the Bragg section 4.

This monolithic component 1 has a substrate 6 n-doped or semi-insulating InP SI common to the different sections 2, 4 and 5. This substrate 6 is electrically connected at neutral voltage.

Amplifier section 2 includes a layer of confinement in InP-n type semiconductor material, one active layer 7 in semiconductor material, such as Solid or quantum well GaInAsP, deposited on InP-n and covered with a layer 8 of p doped InP. On the face of layer 8 which is opposite to substrate 6 are deposited a contact layer 8 ′ of material such as GaInAs and a electrode 9, which supplies the active layer 7 by an amplification current.

The waveguide 3 is made of an organic material and is deposited on a thin anti-reflective layer 10 carried by the substrate 6 and on the face of the amplifying section.

• une couche organique 11 de confinement, de 1,5 µm d'épaisseur, déposée sur la couche antireflet 10, cette couche 11 présentant un indice de réfraction n1 ;
• une couche organique 12 de 1 µm d'épaisseur, constituant le coeur du guide 3, cette couche 12 étant déposée sur la couche 11 et présentant un indice de réfraction n2 supérieur à n1 ;
• une couche organique 13 de confinement supérieur, qui recouvre les couches 12 et 11, cette couche organique 13 de confinement supérieur présentant un indice de réfraction n3 inférieur à n1 et une épaisseur variant entre 1,5 et 2 µm.

More particularly, this waveguide 3 is defined by:

• an organic confinement layer 11, 1.5 μm thick, deposited on the anti-reflection layer 10, this layer 11 having a refractive index n1;
• an organic layer 12 1 μm thick, constituting the core of the guide 3, this layer 12 being deposited on the layer 11 and having a refractive index n2 greater than n1;
• an organic layer 13 of upper confinement, which covers layers 12 and 11, this organic layer 13 of upper confinement having a refractive index n3 less than n1 and a thickness varying between 1.5 and 2 μm.

Layer 12 organic materials are advantageously chosen from polystyrenes (poly (methyl methacrylate) for example), polysiloxanes, polysulfones, fluorinated polyimides, deuterated polyimides (partial substitution of atoms hydrogen by deuterium atoms). These different materials are transparent for a wide range of wavelengths less than 1.6 µm.

Layer 12 has at section 5 of phase control an unetched part 12a and at the level from section 4 of Bragg an engraved part 12b, which defined with layer 13, the network constituting the Bragg reflector.

The face of the layer 13 opposite the substrate 6 door, at Bragg 4 section and section control phase 5, a ribbon 14 and a ribbon 15 forming heating resistors.

These ribbons 14 and 15 are deposited on a layer dielectric 16 interposed between said tapes and the layer 13.

These ribbons 14 and 15 forming resistors of heaters are each controlled by a pair of electrodes 14a, 15a (only one electrode of each pair being shown in the figures). The two electrodes 14a, 15a of the same pair are arranged on either side of the waveguide 3. The voltages controlled by through these electrodes across these ribbons 14 and 15 are independent and are a function of temperatures that we wish to impose in sections 4 and 5.

The thermooptical effect is thus controlled refractive indices of organic guide material wave 3, on the one hand, at the level of section 5 and, on the other hand, at the level of section 4, that is to say the phase and wavelength of the light emitted.

In particular, the variation in Bragg wavelength λB that a temperature variation ΔT allows is given by: ΔλB / λB = Γ (dn / dT) ΔT, where Γ (dn / dT) is the confinement coefficient of the guide, i.e. for an emission wavelength of 1.55 µm, for a temperature variation ΔT of 80 ° C, and for Γ = 0.8 and dn / dT = 2.10 ^-4 , a tunability of 12.6 nm.

The power required for this temperature variation is: P = V 2 / R = Ldk (1 + 0.88e / d) e ΔT where V is the voltage across the heating resistor 14, where R is the ohmic value of this resistance, L and d are its length and width, e is the total thickness of the organic layer and k is the thermal conductivity of the medium organic.

For the example shown in FIGS. 1 and 2, we have:
L = 100 µm, d = 50 µm, e = 4 µm, ΔT = 80 ° K, k = 0.17 W / (m. ° K) and R = 5 kΩ,
which corresponds to a power P of 1.8 mW and a voltage V of 3 Volts.

1) On réalise un empilement de couches semi-conductrices correspondant à la structure de la section amplificatrice 2 par croissances épitaxiales sur le substrat 6, en mettant en oeuvre les techniques de croissances épitaxiales classiques : épitaxie liquide, épitaxie vapeur aux organométalliques (OMCVD), épitaxie par jet moléculaire (MBE), la structure de la couche active pouvant être de type massif ou à puits quantiques.

2) On réalise la structure latérale de la section active 2, de façon à obtenir une structure de type laser enterré, en mettant en oeuvre sur l'empilement obtenu un traitement de gravure, puis une reprise d'épitaxie (croissance de la couche 8 et de la couche 8').

3) On amincit le substrat et on métallise les deux faces de l'échantillon obtenu.

4) On réalise un sillon transversal de 4 à 5 µm de profondeur dans les couches semi-conductrices, par masquage et gravure jusqu'au substrat 6, en utilisant une méthode de gravure sèche (par exemple des techniques de gravure du type de celles classiquement connues sous les dénominations RIE, RIBE, CAIBE), de façon à obtenir dans le semi-conducteur une face lisse et verticale qui constitue la face de sortie de la section amplificatrice 2.

5) On dépose dans le fond du sillon ainsi réalisé et sur la face de sortie de la section amplificatrice la couche antireflet 10, qui est en un matériau diélectrique, tel que SiO ou TiO2, l'épaisseur de cette couche 10 étant de λ/4n, où λ est la longueur d'onde centrale d'émission de la section amplificatrice 2, n est l'indice de réfraction dudit matériau diélectrique tel que n = √nsn2, où ns est l'indice de réfraction du semi-conducteur formant la zone active.

6) On dépose ensuite sur toute la surface du sillon, par centrifugation par exemple, la couche 11 organique de confinement d'épaisseur 1,5 µm (par exemple le PMHA polyméthylméthacrylate).

7) Par gravure (photographie aux ultraviolets profonds dans le cas où le matériau organique est le PMMA), on dégage la face verticale de sortie de la section active 2 et le haut de la section 2.

8) On réalise un fluage du matériau organique déposé par élévation de température (200°C pendant 20 minutes, pour le PMMA) de façon à uniformiser l'épaisseur dudit matériau dans le fond du sillon.

9) On dépose par centrifugation la couche organique d'indice n2 correspondant au coeur 12 du guide (par exemple le polystyrène).

10) On réalise une gravure sèche (RIE oxygène) à travers un masquage métallique ou diélectrique dans les deux couches organiques ainsi déposées, de façon à définir la largeur du guide (2 µm), le ruban organique ainsi réalisé étant dans le prolongement exact de la couche active 7.

11) On réalise le réseau de Bragg sur ce ruban organique, à une distance donnée de la face de sortie de la section active 2. Ce réseau de Bragg peut être par exemple réalisé par gravure directe à travers un masquage holographique classique, ou par inscription photoréfractive dans le guide consistant à irradier la partie supérieure du guide par un système de franges d'interférences, ou encore par écriture photochimique à l'aide d'un laser.

12) On dépose par centrifugation sur tout le sillon la couche de confinement optique supérieure 13 (par exemple le téflon (marque déposée) amorphe ou le matériau commercialisé sous la dénomination commerciale polymatrif, ou du PMMA dans le cas où le matériau de coeur est du polysulfone).

13) On dépose sur toute la surface du matériau organique, une couche diélectrique de protection 16 en un matériau tel que SiO₂ ou Si₃N₄.

14) On dépose au droit des parties 12a et 12b les couches 14 et 15, formant résistance de chauffage. Ces couches 14 et 15 étant par exemple des couches d'une épaisseur de l'ordre de 1000 Å d'or, de platine, de titane ou de chrome.

The manufacturing of the laser component 1 which has just been described is carried out by implementing the following succession of steps:

1) A stack of semiconductor layers corresponding to the structure of the amplifying section 2 is produced by epitaxial growths on the substrate 6, by implementing the conventional epitaxial growth techniques: liquid epitaxy, organometallic vapor epitaxy (OMCVD), Molecular jet epitaxy (MBE), the structure of the active layer possibly being of massive type or with quantum wells.

2) The lateral structure of the active section 2 is produced, so as to obtain a structure of the buried laser type, by implementing on the stack obtained an etching treatment, then a resumption of epitaxy (growth of the layer 8 and layer 8 ').

3) The substrate is thinned and the two faces of the sample obtained are metallized.

4) A transverse groove 4 to 5 μm deep is made in the semiconductor layers, by masking and etching to the substrate 6, using a dry etching method (for example etching techniques of the type of those conventionally known under the names RIE, RIBE, CAIBE), so as to obtain in the semiconductor a smooth and vertical face which constitutes the output face of the amplifying section 2.

5) The anti-reflective layer 10, which is made of a dielectric material, such as SiO or TiO2, is deposited in the bottom of the groove thus produced and on the outlet face of the amplifying section. The thickness of this layer 10 is λ / 4n, where λ is the central emission wavelength of the amplifying section 2, n is the refractive index of said dielectric material such that n = √nsn2, where ns is the refractive index of the semiconductor forming the active area.

6) The organic layer 11 of 1.5 μm thick confinement layer (for example PMHA polymethylmethacrylate) is then deposited over the entire surface of the groove, by centrifugation for example.

7) By etching (deep ultraviolet photography in the case where the organic material is PMMA), the vertical exit face of the active section 2 and the top of the section 2 are released.

8) A creep of the organic material deposited is carried out by raising the temperature (200 ° C. for 20 minutes, for PMMA) so as to standardize the thickness of said material in the bottom of the groove.

9) The organic layer of index n2 corresponding to the core 12 of the guide (for example polystyrene) is deposited by centrifugation.

10) A dry etching (RIE oxygen) is carried out through a metallic or dielectric masking in the two organic layers thus deposited, so as to define the width of the guide (2 μm), the organic ribbon thus produced being in the exact extension of the active layer 7.

11) The Bragg grating is produced on this organic ribbon, at a given distance from the exit face of the active section 2. This Bragg grating can for example be produced by direct etching through conventional holographic masking, or by writing photorefractive in the guide consisting in irradiating the upper part of the guide by a system of interference fringes, or even by photochemical writing using a laser.

12) The upper optical confinement layer 13 (for example Teflon (registered trademark) amorphous or the material sold under the trade name polymatrif, or PMMA in the case where the core material is polysulfone).

13) A protective dielectric layer 16 made of a material such as SiO ₂ or Si ₃ N ₄ is deposited over the entire surface of the organic material.

14) The layers 14 and 15 are deposited at the right of the parts 12a and 12b, forming a heating resistor. These layers 14 and 15 being for example layers with a thickness of the order of 1000 Å of gold, platinum, titanium or chromium.

The sample thus obtained is then cut into several chips forming the component represented on the figure 1.

Alternatively, as illustrated in Figure 3, the Bragg grating can be formed not by etching of layer 12, but by means of a network of resistors heaters 14b replacing the resistor 14. This network thermooptically induced a network of index of refraction at the level of the organic waveguide, the coupling coefficient will depend on the applied voltage on the heating resistors 14b.

Of course, the structure that has just been described can be integrated into components monolithic having other functions than the only laser emission function.

In particular, as illustrated on the Figure 4, this structure can be integrated into a organic filter laser component comprising, on the same substrate 22, a directional coupler 20 with waveguides organic and a DBR 21 laser, of the type described in reference to FIGS. 1 to 3, the Bragg section of which is extended by one of the branches of the coupler 20. On the Figure 4, the active section of the laser 21 has been referenced by 21a, the Bragg section by 21b.

Also, as illustrated in FIG. 5, the organic DBR lasers of the type described with reference to Figures 1 to 3, can be integrated to constitute bars 30 of N lasers 31. The sections of Bragg 31a lasers 31 are advantageously extended by organic material waveguides of a coupling section 32 to N inputs. In Figure 5, this section 32 constitutes a combiner with N inputs and a exit.

Other optical functions are of course possible at the output of the bar. Section 32 can particular be replaced by a switching matrix with N inputs and N output, consisting of N couplers directives.

Advantageously, during the manufacture of such components integrating DBR lasers and other functions, the organic layers of the DBR lasers and those of the waveguides which the one or them extend.

Other variants of the invention are indeed heard possible. In particular, the heart of the guide wave may be made of an electro-optical organic material with nonlinear optical property (NLO) (such as by doped or grafted polyimides), the component DBR laser comprising instead of heating of the component of FIGS. 1 and 2, of the electrodes for applying an electric field to the waveguide intended to vary the refractive index of its heart.

Claims (11)

1. A laser component having a Bragg reflector, the component comprising a substrate (6) carrying firstly an active emission section (2) of semiconductor materials and secondly a Bragg reflector waveguide (3) extending said active section (2), means (14. 15. 14a, 15a) enabling the refractive index of the waveguide (2) to be varied so as to control the wavelength tuning of said laser component. characterized in that said waveguide (3) is made of organic material(s).
2. A component according to claim 1. characterized in that the waveguide (3) is made of thermo-optical organic material(s), and in that the means (14. 15. 14a, 15a) enabling the refractive index of the waveguide to be varied comprise means for heating the Bragg reflector section thereof.
3. A component according to claim 2, characterized in that it includes an array of heating resistances (14b) which induce an array of refractive indices on the waveguide.
4. A component according to claim 2 or 3, characterized in that the organic material(s) of the waveguide (3) is/are of low thermal conductivity. preferably about or less than 1 W/(m.°K) and a large coefficient of refractive index variation as a function of temperature. preferably greater than 10^-4°C^-1.
5. A component according to claim 1, characterized in that the core of the waveguide is made of an electro-optical organic material and in that the means enabling the refractive index of the waveguide to be varied comprise means for applying an electric field to said waveguide.
6. A component according to any preceding claim, characterized in that the organic material(s) is/are selected from: polystyrenes; polysiloxanes: fluorine-containing polyimides; and deuterium-containing polyimides.
7. A component according to any preceding claim, characterized in that the organic waveguide (3) includes a grating-free portion (5) interposed between the Bragg section (4) and the active section (2), said grating-free portion corresponding to the phase control section of the component.
8. A monolithic component integrating at least one Bragg reflector laser component (21. 31) according to any preceding claim.
9. A monolithic component according to claim 8, characterized in that at least one integrated organic light waveguide extends the Bragg section of the laser component.
10. A method of making a component according to any one of claims 1 to 9. characterized by the following steps:

    a stack of semiconductor layers corresponding to the structure of the active section (2) is made on the substrate (6) by epitaxial growth;

    the lateral structure (8) of said active section is made by etching and repeating epitaxy;

    a furrow is made in the resulting structure by masking and dry etching down to the substrate (6);

    an antireflection layer (10) of dielectric material is deposited on the bottom and the sides of the furrow, and also on the outlet face of the active section;

    a confinement layer (11) of organic material having a refractive index smaller than that of the core of the waveguide is deposited on said antireflection layer (10);

    a layer of organic material corresponding to the core (12) of the waveguide is deposited on said confinement layer (11);

    a ridge is etched in the organic layers (11. 12) deposited in this way;

    where appropriate, the Bragg grating is made on the ridge corresponding to the core (12) of the waveguide;

    a top confinement layer (13) of an organic material having a refractive index that is smaller than that of the core of the waveguide is deposited on the ridge and on the antireflection layer (10);

    a dielectric protection layer (16) is deposited on said confinement layer; and

    layers forming heating resistances (14. 14b. 15) are deposited on said dielectric layer (16).
11. A method according to claim 10, characterized in that after the first organic confinement layer has been deposited. said layer is etched to clear the outlet face of the active section.

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